The present invention relates to an improved method of sludge concentration and more particularly pertains to a sludge flotation method that uses less flocculating agent than is required in conventional flotation methods.
Flotation methods are extensively used for sludge concentration. According to a pressurized flotation concentration method, clarified water having air dissolved at a pressure of 4-5 kg/cm.sup.2 is mixed with the sludge, and the resulting fine bubbles are used to cause concentrated flocs of solids to float on the surface of the liquid mixture. This method not only requires high power consumption for pressurizing air but also involves much difficulty in achieving high sludge concentration since the air bubbles are bonded to the sludge solids merely by weak physical adsorption.
Commonly assigned Japanese Laid-Open Patent Application No. 40499/81 discloses an atmospheric flotation method as a new technique for concentrating sludge without incurring the disadvantages of the conventional method. According to this new method, air is introduced at atmospheric pressure into a liquid phase containing foaming agent and flocculating agents so as to evolve a froth, which is mixed with the sludge, and the mixture is sent to a flotation tank for concentrating the sludge solids by flotation. This method is superior to the pressurized flotation technique in that it provides an inexpensive and simple process for achieving a high concentration of sludge.